Various kinds of gypsum boards having a gypsum core covered with paper sheets are generally in practical use for architectural interior finish work or the like. In general, a process of producing such a gypsum board includes steps of admixing a quantity of water and foam with ingredients for the gypsum core, such as calcined gypsum, an adhesion promoting agent, a set accelerator, additives, intimate mixtures and the like, so as to produce a gypsum slurry; disposing the gypsum slurry between paper cover sheets to form a continuous web of gypsum board; generally shaping and setting the web in a predetermined outline of a board; and further, severing and drying it and finally cutting the boards in a predetermined size of the gypsum board product.
A thin, circular mixer is generally used as a mixer for admixing the aforementioned materials for the gypsum boards. This kind of mixer comprises a flattened cylindrical housing or casing, and a rotary disc to be rotated within the housing in operation of the rotary drive means. An upper cover plate of the housing is provided at its central area with a plurality of inlet ports for introducing the feedstock materials into the housing and the housing is provided in its peripheral zone with a slurry outlet port for discharging the gypsum slurry therethrough.
In the conventional mixers, the rotary disc is formed with a serrated or toothed peripheral edge for displacing the gypsum slurry radially outward. On the rotary disc, a plurality of lower pins are fixed to project therefrom, which define movable pins, whereas a plurality of upper pins are supported by the upper cover plate to depend therefrom, the upper pins defining stationary pins. The lower and upper pins are alternately arranged in a radial direction of the disc and are moved relative to each other during rotation of the disc to cooperate with each other so as to agitate and admix the ingredients with the water in the housing. The rotating disc allows the gypsum slurry to be discharged through the slurry outlet port so as to be fed to the following slurry-disposing step.
This kind of pin mixer is disclosed, e.g., in Japanese Patent Laid-Open Publication No. 8-25342. FIG. 9 is a fragmentary perspective view, partly broken away, illustrating an internal structure of the pin mixer, and FIGS. 10(A) and 10(B) are side elevational and transverse cross-sectional views illustrating structural arrangements of lower and upper pins as shown in FIG. 9.
As shown in FIG. 9, the rotary disc D located within the housing H has the peripheral edge with a plurality of tooth elements G, the respective tooth elements G being circumferentially spaced apart a constant distance. The lower pin P1, which is fixed on the disc to protrude therefrom vertically upward, is configured in the form of round column with a uniform cross-sectional profile throughout its entire height. The upper cover plate C of the housing H is provided with the upper pins P2 fixed thereto and depending therefrom, which are configured in a form of a round column substantially identical with that of the lower pin P1.
During rotation of the disc D, the lower pin P1 moves in the direction of rotation R, and the front surface thereof facing forward as seen in the rotational direction R propels and displaces the gypsum slurry so that a counter-flow of the slurry or ingredients relative to the pin P1 is formed to move backward with respect to the rotational direction R. A vortex or turbulent flow zone, which represents a transitional retentive state of the slurry, is provided behind the pin P1, so that the slurry to be fluidized by mixing action tends to deposit on and adhere to the rear surface of the pin. The deposition of the slurry behind the pin P1 develops or grows gradually as the mixing and agitating operation is continuously in progress. The set accelerating action of the set accelerator contained in the ingredients affects and additionally promotes such a deposition of slurry, and thus, a relatively large mass S of the slurry is formed on the rear surface of the pin P1 as shown in FIG. 10.
An analogous condition can be observed in the upper pin P2 as shown in FIG. 10(A), and also, it appears on the toothed elements G in the toothed peripheral edge of the disc D. The recess or cavity formed between the respective tooth elements G, which is a so-called "dead space", temporarily receives the slurry and acts to propel or displace the slurry into the slurry outlet port. The slurry retentively trapped in the dead space, however, tends to set therein and deposit on the tooth element G. The deposition of slurry in the dead space further develops or grows, owing to the set accelerating action of the set accelerator or the like, so that a relatively large solid mass of slurry adheres to the tooth element.
These kinds of solid mass deteriorate the fluidity of the ingredients and slurry within the mixer, and degrades the mixing performance of the mixer, and an excessive growth of the solid mass of slurry during continuous operation of the mixer may result in an irregular load distribution of the disc, which may generates microvibration of the disc accompanied by partial removal or separation of the solid mass. The removed pieces or sections are fed to the following slurry-disposing step together with the gypsum slurry to be contained in the gypsum core of the gypsum board. In the gypsum board containing such foreign matter or impurities, a failure or defectiveness of product quality, such as a local depression or recess on the gypsum board, is apt to appear on the surface of the board product, and this kind of failure results in a degradation of the production efficiency or actual yield of production. Thus, an improvement for surely preventing such a failure is desired.
It is an object of the present invention, therefore, to provide a pin mixer having a simplified arrangement that surely prevents a solid mass of slurry from depositing on a pin or a toothed edge of a rotary disc.